A conventional water base dual color ink composition contains a metal powder pigment such as aluminum powder, a water-soluble dye, water, and a penetrating organic solvent (see patent literatures 1 to 3). When a writing is provided using the above-mentioned ink composition on a porous substrate such as paper into which the penetrating organic solvent penetrates or which can absorb the solvent, the metal powder pigment is immediately trapped in situ on the substrate with which the pigment has come into contact to form a metalescent inner portion as written, and the dye penetrates and diffuses into the substrate around the inner portion together with the penetrating organic solvent, and it exudes from the inner portion to form outer contour portions around the inner portion, thereby forming a dual color writing, that is, a writing with border or fringe. Such a dual color ink composition may be used as an ink composition for marking pens, felt pens, water base ballpoint pens, and the like.
However, as the first disadvantage of the conventional water base dual color ink compositions, the following is noted. In order to improve the fixability of writing, a carboxymethyl cellulose sodium salt or a water-soluble resin such as polyvinyl alcohol is used as a fixing agent (see patent literatures 1 and 2). However, when a little larger amount of the fixing agent is used to make the viscosity of the resulting water base dual color ink composition relatively higher so that it can be used as, for example, an ink composition for a water base ballpoint pen, the fixing agent in the ink composition often hinders the formation of contour portions of writing when the ink composition is used, and thus the ink composition fails to provide desirable dual color effect.
It has been already known that in common ink compositions, which are not dual color compositions, a pearlescent pigment is used as a pigment as well as a water-soluble rosin or a styrene-acrylic acid copolymer is used as a fixing agent (see patent literature 4). These fixing agents also hinder the formation of contour portions when used for the water base dual color ink composition.
The second disadvantage is as follows. In conventional dual color ink compositions, an inner portion of writing is formed by a pigment under no influence of color of a dye used, while a metal powder pigment which has strong hiding power is used so that the resulting contour portions of writing are clearly distinguished from the inner portion. When using such a dual color ink composition, the inner portion of writing has the same color as the color the metal powder pigment originally has, regardless of a dye used, but the gloss is undeniably unsatisfactory. Then, a water base dual color ink composition which contains, in addition to or instead of metal powder pigment, a pearlescent pigment as a pigment has recently been proposed (patent literature 5).
However, conventional dual color ink compositions prepared using a pearlescent pigment have a defect that they hardly exhibit clear dual color effect because the pearlescent pigment has small hiding power than a metal powder pigment, and hence the inner portion of writing formed is influenced by the color of a dye used.
Also in conventional dual color ink compositions using a pearlscent pigment, when outer contour portions of writing are to be formed by a pale color dye, of course, the inner portion of writing is so formed as to have almost the same color as that the pearlescent pigment originally possesses, while the outer contour portions are to an extent so formed as to have a pale color, even if the pearlescent pigment has small hiding power.
However, when a deep color dye such as black, blue, brown or deep green dye is used in conventional dual color ink compositions containing a pearlescent pigment to form deep color contour portions of writing, the deep color contour portions are in fact clearly formed by such a dye, but the resulting pearlescent inner portion of writing is hardly distinguished visually clearly from the deep color contour portions under the influence of the deep color dye, thereby the ink composition hardly provides clear dual color effect. In other words, the deep color dye exerts an adverse effect to the color development of the pearlescent pigment to form the inner portion, thereby the inner portion cannot be clearly formed by the pearlescent pigment.    Patent literature 1: Japanese Unexamined Patent Publication No. 60-231777    Patent literature 2: Japanese Unexamined Patent Publication No. 61-123684    Patent literature 3: Japanese Unexamined Patent Publication No. 2000-129188    Patent literature 4: Japanese Unexamined Patent Publication No. 10-245517    Patent literature 5: Japanese Unexamined Patent Publication No. 2007-31558